Muscle
tissues

We know that living organisms can move
on their own or can perform other types
of movement. Muscle tissue has a ability
to relax and contrast and so
bring about movement and mechanical
work in various parts of the body.
There are other movements in the body too
which are necessary for the survival of
the organism such as the heart beat
and the movements of the alimentary
canal.

Muscles can be divided into three main
groups according to their structure,
e.g.:

Smooth muscle tissue.

Skeletal muscle tissue.

Cardiac (heart) muscle tissue.

A whole
muscle with many
fasciculi

Types
of Muscle Tissue

Smooth
muscle tissue is made up of thin-elongated
muscle cells, fibres.
These fibres are pointed at their
ends and each has a single,
large, oval nucleus. Each
cell is filled with a specialised
cytoplasm, the sarcoplasm and
is surrounded by a thin cell
membrane, the sarcolemma.
Each cell has many myofibrils
which lie parallel to one another
in the direction of the long axis
of the cell. They are not
arranged in a definite striped
(striated) pattern, as in
skeletal muscles - hence the
name smooth muscle . Smooth
muscle fibres interlace to form sheets
or layers of muscle tissue
rather than bundles.
Smooth muscle is involuntary
tissue, i.e. it is not
controlled by the brain. Smooth
muscle forms the muscle layers in
the walls of hollow organs
such as the digestive tract
(lower part of the oesophagus,
stomach and intestines), the
walls of the bladder, the uterus,
various ducts of glands
and the walls of blood vessels
.

Functions
of Smooth Muscle Tissue

Smooth muscle controls
slow, involuntary
movements such as the
contraction of the smooth
muscle tissue in the
walls of the stomach and
intestines.

The muscle of the
arteries contracts and
relaxes to regulate the
blood pressure and the
flow of blood.

Skeletal
muscle is the most abundant
tissue in the vertebrate
body. These muscles are attached
to and bring about the movement
of the various bones of the
skeleton, hence the name skeletal
muscles. The whole muscle,
such as the biceps, is enclosed
in a sheath of connective tissue,
the epimysium. This sheath
folds inwards into the substance
of the muscle to surround a large
number of smaller bundles, the fasciculi.
These fasciculi consist of still
smaller bundles of elongated,
cylindrical muscle cells, the fibres.
Each fibre is a syncytium,
i.e. a cell that have many
nuclei. The nuclei are
oval in shaped and are found at
the periphery of the cell, just
beneath the thin, elastic
membrane (sarcolemma). The sarcoplasm
also has many alternating light
and dark bands, giving the
fibre a striped or striated
appearance (hence the name striated
muscle). With the aid of an
electron microscope it can be
seen that each muscle fibre is
made up of many smaller units,
the myofibrils. Each
myofibril consists of small
protein filaments, known as actin
and myosin filaments. The myosin
filaments are slightly thicker
and make up the dark band (or
A-band). The actin
filaments make up the light
bands (I-bands) which are
situated on either side of the
dark band. The actin filaments
are attached to the Z-line.
This arrangement of actin and
myosin filaments is known as a sacromere.

A
myofibril with actin and
myosin filaments

During the
contraction of skeletal muscle
tissue, the actin filaments slide
inwards between the myosin
filaments. Mitochondria provide
the energy for this to take
place. This action causes a shortening
of the sacromeres (Z-lines move
closer together), which in
turn causes the whole muscle
fibre to contract. This
can bring about a shortening
of the entire muscle such as
the biceps, depending on the
number of muscles fibres that
were stimulated. The contraction
of skeletal muscle tissue is very
quick and forceful.

Functions
of Skeletal Muscle Tissue

Skeletal muscles
function in pairs to
bring about the co-ordinated
movements of the limbs,
trunk, jaws, eyeballs,
etc.

This is a unique tissue found only
in the walls of the heart.
Cardiac (Heart) Muscle Tissue
shows some of the characteristics
of smooth muscle and some of skeletal
muscle tissue. Its fibres ,
like those of skeletal muscle,
have cross-striations and
contain numerous nuclei.
However, like smooth muscle
tissue, it is involuntary.
Cardiac muscle differ from
striated muscle in the
following aspects: they are shorter,
the striations are not so
obvious, the sarcolemma is
thinner and not clearly
discernible, there is only one
nucleus present in the centre
of each cardiac fibre and adjacent
fibres branch but are linked
to each other by so-called muscle
bridges. The spaces between
different fibres are filled with areolar
connective tissue which
contains blood capillaries to
supply the tissue with the oxygen
and nutrients.

Functions
of Cardiac (Heart) Muscle Tissue

Cardiac muscle tissue
plays the most important
role in the contraction
of the atria and
ventricles of the heart.

It causes the
rhythmical beating of the
heart, circulating
the blood and its
contents throughout the
body as a consequence.